On Swift Wings

IIn 2006 when the U.S. military retired the F-14 Tomcat—of Top Gun fame—the most successful experiment in “swing-wing” airplanes was also grounded. Such planes rely on an intuitively appealing design idea: change the shape of the wing in flight and you can optimize the plane’s performance over a range of speeds. Unfortunately, even as the best of its kind, the Tomcat was a poor imitation of its biological inspiration—birds that morph their wings in flight. It’s been hard, though, for engineers to disentangle the wing changes in birds that are aerodynamically advantageous from the changes a wing must make for flapping. Now, thanks to an elegant study of swifts, investigators have their first empirical look at the effects of variable wing shape on bird flight.

Swifts belong to the family Apodidae, which literally means “without feet” in Latin. Footless may be a bit of an exaggeration, but it’s pretty near the mark. The birds’ tiny feet seldom touch down because feeding, courting, and even sleeping all take place on the fly. Because swifts glide for long distances—without complicating things for observers by flapping their wings—and because they change the geometry of their wings in flight, they make an ideal bird with which to study the advantages of wing morphing.

Overall, the swift wing resembles a long, thin, curved blade that tapers to a sharp point, much like that of a scythe. Structurally, the wing closely resembles your arm: remove or fuse a few bones, add some feathers, and you just about have it. When the wings flap, as when your arms do, chest muscles power their motion.

In essence, those sets of bones give swifts, and all flying birds, two feathered airfoils on each wing, crucial to flapping in flight. One airfoil is made up of primary flight feathers attached to the wingtip bones. The other is formed by the secondary flight feathers attached to the forelimb bones. The swift, however, gains added maneuverability by having an unusually large proportion of its wing made up of the “hand,” or wingtip, bones, compared with typical birds. By changing the “wrist” angle between the “hand” and forelimb, the swift can change both the shape of the wing and its area [see illustrations left].

David Lentink, a biomechanist at Wageningen University in the Netherlands, and a team of aerodynamicists from the Netherlands and Sweden measured swift wings in a range of natural flight configurations. One of the most important factors, they discovered, is the aforementioned angle between the hand and the forelimb. Folding the wrist knocks a whopping 30 percent off the wing area.

Lentink and his colleagues also attached wings (from birds that had died in sanctuaries) to a finely calibrated balance in a wind tunnel, to measure how various wing geometries alter drag and lift. Drag is the force that acts in the same direction as the airflow, whereas lift acts perpendicular to airflow. Drag and lift, along with a bird’s mass, determine the biologically important variables of gliding flight. For swifts with unfolded wrists and outstretched wings flying in a straight line, Lentink and his colleagues determined that a speed of about twenty miles an hour gives the birds the maximum gliding time and the steadiest flight path for the least energy. So even though outstretched wings provide more drag, the wide spread gives a lot more lift for level flights.

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[media:node/942 horizontal right caption medium]That fits well with what is known about swift “roosting” behavior: tracking the birds on radar as they sleep on the wing shows that they glide and periodically flap to maintain an average speed of twenty miles per hour, and to minimize altitude lost per minute. In other words, they glide at a speed that requires minimum energy during their snooze time.

At high speeds, however, the advantage swiftly shifts to swept-back wings. Swifts are called swifts, after all, because their peak flight speed is so high. They have been clocked at more than sixty miles an hour—no wonder they fall asleep at twenty! It turns out that sweeping the wings back becomes key to energy efficiency at fast cruising speeds of more than forty miles an hour. At those higher speeds, the swept wings, which minimize drag, become more efficient both in maintaining a shallow-angle glide and in maximizing the time spent aloft.

If straight-line flying benefits from spread wings at lower speeds and swept wings at higher speeds, what about turning? The F-14 Tomcat spread its wings wide for increased maneuverability during dogfights, which demand lots of fast turns. Keeping the spread-wing shape when turning is also a good strategy for the swift—at least at twenty miles an hour. At that velocity, a swift can turn at least twice as fast by spreading its wings as it can by bending them.

In higher-speed turns, however—as simulated in the wind tunnel—it becomes impossible to measure the relative efficiency of spread and swept wings, because beginning around thirty-four miles an hour, spread swift wings become vulnerable to damage from aerodynamic stress. At high speeds they naturally flex and twist slightly in the turbulent air, and so avoid being damaged by the high forces. So, when pursuing fast-flying and fast-turning insect lunches, a swift bends its wings to keep up; the calories from such a meal warrant the extra effort.

Human attempts at variable wing geometry have always been hampered by the complexity and weight associated with a system engineered from hinges. Students working with Lentink are now building flexible, lightweight aircraft the size of swifts, capable of wing morphing. If such devices can be scaled up, fighter pilots could have a fast, agile plane that could slow down and spread its wings to hold station and maximize its time in the air. Even then, though, the pilots probably won’t be dozing off.